1. Field of the Invention.
The present invention relates generally to a pitch trim and control apparatus and method, and more particularly to a method and apparatus for providing full elevator travel in a system which uses a movable horizontal stabilizer having an extreme range of movement.
2. Description of the Prior Art.
Aircraft pitch trim per se has been recognized for many years. As is well known, the flight characteristics of an aircraft vary according to speed, power setting, etc. Accordingly, it has been the practice to provide trim mechanisms to, under a given set of conditions, afford neutral forces on the pilot's controls. As the operating parameters are altered, the trim mechanisms may be also altered to compensate and again provide the desired, neutral control forces.
Examples of such conventional trim mechanisms are to be found in U.S. Pat. No. 2,094,488 issued Sept. 28, 1937; U.S. Pat. No. 3,207,458 issued Sept. 21, 1965; and U.S. Pat. No. 3.109,614 issued Nov. 5, 1963.
A conventional mechanism for pitch trim comprises a horizontal stabilizer member pivoted around an axis transverse to the airplane fuselage for, nominally, vertical movement. Transient pitch control is provided by an elevator member pivotally attached along an axis parallel to the axis of rotation of the horizontal stabilizer and movable up and down for desired pitch attitude corrections inputs. As illustrated by the above mentioned patents, it has been recognized for some time that pitch trim inputs to the horizontal stabilizer effect the relationship of the elevator relative to the horizontal stabilizer and, accordingly, require compensation.
Another factor of concern is the unusual conditions attendant to a slow flying aircraft, such as a STOL aircraft, in which lift may be augmented by means other than the velocity of the aircraft through the ambient air. For instance, upper wing surface blowing or externally blown flaps develop very high lift coefficients at relatively low aircraft velocities. These lift generating means, however, generate large angles of down wash behind the wing. Thus, the velocity of the slip stream inpinging upon the horizontal stabilizer and elevator is at a relatively high angle to the longitudinal axis of the aircraft. Accordingly, to prevent the horizontal stabilizer from stalling, it is necessary to substantially adjust the angle of the horizontal stabilizer relative to the fuselage to that of a relatively high angle. The adjustments in the horizontal stabilizer angle of incidence are substantially greater than the minor trim angles incurred in conventional aircraft. In fact, the adjustments are often so great as to seriously compromise the travel of the elevator as a result of such adjustments. Conventional pitch trim configurations are inappropriate for such extreme setting and for the unusual airflow conditions.